1. Field of the Invention
This invention relates generally to the concepts of forming metallic materials and improving the properties of these materials, and more particularly to an improved method of improving the properties of superplastically formed metallic materials, such as aluminum alloys, by healing the cavities that are formed in the structure of these materials as a result of a superplastic forming technique.
2. Description of the Prior Art
Superplastic forming is one of the most important metal forming technologies to emerge in recent years. This technology uses a phenomenon called superplasticity which occurs in several metals under certain conditions of microstructure, temperature, and strain rate. The most important characteristic of superplastically formable materials is their exceptional stability in uniaxial tensile deformation. This enables them to develop extremely large elongations, usually greater than 200 percent, without fracture; whereas, for conventional materials the equivalent values are usually less than 50 percent. Since the potential for large elongations in several structural metals, such as titanium and aluminum alloys, was first demonstrated, the superplastic forming of such alloys has been systematically developed into a technology for manufacturing parts on a production basis.
There are many methods described in the prior art that employ the principle of superplastic metal forming. However, many of these superplastic forming techniques result in the disadvantage of forming internal voids or cavities at the grain boundary area of the materila, i.e. "cavitation", which generally results in poorer mechanical properties in the metal part so formed.
U.S. Pat. No. 4,288,021 describes tooling for use in an autoclave or a hot press for the superplastic forming and diffusion bonding of metals. An autoclave is used to contain the metal component prior to superplastic forming as well as the entire tooling for the forming procedure. The primary objectives are to provide a method of superplastic forming by employing an effective high-temperature/gas-pressure sealing method and an inert atmosphere for forming. In the described method, the gas is controllably leaked so that the forming part only experiences relatively low pressures (estimated to be about 200 psig from the description), i.e., only the magnitude of pressure necessary for superplastically forming the metal component into a part. While this magnitude of pressure may be sufficient to form the metal component into a part with a specific geometrical shape, it is not sufficient to prevent the formation of cavities in structural high-strength alloys (such as many of the structural aluminum alloys). Furthermore, once the cavities are formed within the material, even higher pressures than those necessary for prevention of cavity nucleation (or generation) will be required for cavity closure. This is a typical example of the type of superplastic forming process that can lead to the problem of cavitation.
U.S. Pat. No. 4,354,369 describes a method for superplastic forming which according to the disclosure eliminates internal voids. In the first described embodiment a blank of material which is capable of being formed superplastically is held opposite a forming surface of a die. The blank is then heated to the superplastic forming temperature and a pressure is applied to both sides of the blank. The pressure is sufficient to prevent the formation of voids. Thus, the primary objective of the method of this patent is to prevent internal voids from occurring during superplastic forming. Should the proposed method not succeed, the patent also has a secondary objective to remove already formed voids by plastic deformation and diffusion. It teaches the use of forming gas pressure to force the formed part against the tool surface to remove voids. Based upon the current industrial pratices of superplastic forming, the peak magnitudes of gas pressure are in the range of about 100 to 500 psig. Currently used tooling is not capable of safe application at pressures much beyond this range. While these pressures are adequate to suppress, even prevent, cavitation during forming, they are grossly inadequate to heal already formed cavities, and thus not a solution to the problem of residual cavitation in the material after forming.